The order of the radiographs, in each stage, was different and random. The results were determined by calculating the inter-observer and intra-observer agreement, and were quantified using two-rater and multi-rater kappa statistics.

The inter-rater agreement in the first, and second, stage was moderate (multi-rater kappa coefficient: 0.41 in the first stage and 0.43 in the second stage; percentage agreement: 0.56 and 0.55 respectively). The inter-rater agreement in the third stage was less (multi-rater kappa coefficient: 0.34 and percentage agreement: 0.47). The intra-observer agreement was the highest between the first and second stage for each of the researchers (mean kappa coefficient: 0.69; mean percentage agreement: 0.76).

The use of lateral bending X-rays in classifying the IS, according to the criteria of Lenke, reduced the intra-rater and the inter-rater agreement. Supine traction radiographs of the spine improved the agreement. This may suggest that the supine traction X-rays may help in classifying the IS.

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